Quote Originally Posted by Kara
But then there is magic ink too. In CoS Harry tries to do a lot of things to make the ink on the pages of Riddle's diary visible, so we have to assume that there are just as many ways of making the ink invisible after the message has been read too. Also when Harry writes on the book, the ink vanishes... right? Or do you think that's just because the diary was a Horcrux? hmm...
There's always the Marauders' Map. That 'spoke' to Snape didn't it. I think communicating via parchment is very feasible.